package org.stan.yxgz.util;

//Copyright 2007 Christian d'Heureuse, Inventec Informatik AG, Zurich, Switzerland
//www.source-code.biz, www.inventec.ch/chdh
//
//This module is multi-licensed and may be used under the terms
//of any of the following licenses:
//
//EPL, Eclipse Public License, V1.0 or later, http://www.eclipse.org/legal
//LGPL, GNU Lesser General Public License, V2 or later, http://www.gnu.org/licenses/lgpl.html
//GPL, GNU General Public License, V2 or later, http://www.gnu.org/licenses/gpl.html
//AL, Apache License, V2.0 or later, http://www.apache.org/licenses
//BSD, BSD License, http://www.opensource.org/licenses/bsd-license.php
//
//Please contact the author if you need another license.
//This module is provided "as is", without warranties of any kind.

import java.util.LinkedHashMap;
import java.util.Collection;
import java.util.Map;
import java.util.ArrayList;

/**
* An LRU cache, based on <code>LinkedHashMap</code>.
*
* <p>
* This cache has a fixed maximum number of elements (<code>cacheSize</code>).
* If the cache is full and another entry is added, the LRU (least recently used) entry is dropped.
*
* <p>
* This class is thread-safe. All methods of this class are synchronized.
*
* <p>
* Author: Christian d'Heureuse, Inventec Informatik AG, Zurich, Switzerland<br>
* Multi-licensed: EPL / LGPL / GPL / AL / BSD.
*/
public class LRUCache<K,V> {

	private static final float   hashTableLoadFactor = 0.75f;
	
	private LinkedHashMap<K,V>   map;
	private int                  cacheSize;

/**
* Creates a new LRU cache.
* @param cacheSize the maximum number of entries that will be kept in this cache.
*/
public LRUCache (int cacheSize) {
	this.cacheSize = cacheSize;
	int hashTableCapacity = (int)Math.ceil(cacheSize / hashTableLoadFactor) + 1;
	map = new LinkedHashMap<K,V>(hashTableCapacity, hashTableLoadFactor, true) {
			   // (an anonymous inner class)
		   private static final long serialVersionUID = 1;
		   @Override protected boolean removeEldestEntry (Map.Entry<K,V> eldest) {
		      return size() > LRUCache.this.cacheSize; }
		   };
	}

/**
* Retrieves an entry from the cache.<br>
* The retrieved entry becomes the MRU (most recently used) entry.
* @param key the key whose associated value is to be returned.
* @return    the value associated to this key, or null if no value with this key exists in the cache.
*/
public synchronized V get (K key) {
return map.get(key); }

/**
* Adds an entry to this cache.
* The new entry becomes the MRU (most recently used) entry.
* If an entry with the specified key already exists in the cache, it is replaced by the new entry.
* If the cache is full, the LRU (least recently used) entry is removed from the cache.
* @param key    the key with which the specified value is to be associated.
* @param value  a value to be associated with the specified key.
*/
public synchronized void put (K key, V value) {
map.put (key, value); }

/**
* Clears the cache.
*/
public synchronized void clear() {
map.clear(); }

/**
* Returns the number of used entries in the cache.
* @return the number of entries currently in the cache.
*/
public synchronized int usedEntries() {
return map.size(); }

/**
* Returns a <code>Collection</code> that contains a copy of all cache entries.
* @return a <code>Collection</code> with a copy of the cache content.
*/
public synchronized Collection<Map.Entry<K,V>> getAll() {
return new ArrayList<Map.Entry<K,V>>(map.entrySet()); }

public static void main (String[] args) {
	   LRUCache<String,String> c = new LRUCache<String, String>(3);
	   c.put ("1", "one");                           // 1
	   c.put ("2", "two");                           // 2 1
	   c.put ("3", "three");                         // 3 2 1
	   c.put ("4", "four");                          // 4 3 2
	   if (c.get("2") == null) throw new Error();    // 2 4 3
	   c.put ("5", "five");                          // 5 2 4
	   c.put ("4", "second four");                   // 4 5 2
	   // Verify cache content.
	   if (c.usedEntries() != 3)              throw new Error();
	   if (!c.get("4").equals("second four")) throw new Error();
	   if (!c.get("5").equals("five"))        throw new Error();
	   if (!c.get("2").equals("two"))         throw new Error();
	   // List cache content.
	   for (Map.Entry<String, String> e : c.getAll())
	      System.out.println (e.getKey() + " : " + e.getValue()); }

} // end class LRUCache


